1. Field of the Invention
The present invention relates to a conveying apparatus, which conveys a sheet, and an image forming apparatus provided with the conveying apparatus.
2. Description of the Related Art
An image forming apparatus, such as a printer, a copying machine or a facsimile, usually has a conveying apparatus adapted to take out one sheet material at a time from a stack of a plurality of sheet materials and convey the sheet. The conveying apparatus has a conveying roller, which is rotatively driven by a drive source, and a pinch roller, which is disposed such that it opposes the conveying roller and which follows the rotation of the conveying roller. The conveying apparatus conveys a sheet material by rotating the conveying roller, with the sheet material sandwiched between the conveying roller and the pinch roller.
Some of the conveying apparatuses include skew correcting mechanisms for correcting the skew of a sheet material relative to the conveying direction of the sheet material. The skew of the sheet material relative to the conveying direction can be corrected by abutting the leading end of the sheet material against a resist roller or a resist shutter. The skew of the sheet material is corrected at an upstream in the conveying direction of the sheet material relative to an image forming portion in which an image is formed on the sheet material.
When the skew of the skewed sheet material is corrected, the sheet material is warped between a member for correcting the skew and a conveying mechanism, generating a rotational force about the normal line of the paper surface is in the sheet. In other words, there are cases where the leading end of the sheet material, the skew of which has been corrected, is aligned in a direction orthogonal to the conveying direction, whereas a pair of rollers conveying the sheet material is sandwiching the sheet material still in a skewed manner. In some cases, therefore, a problem is posed that the sheet material which has undergone the correction of the skew is skewed again by the rotational force.
According to the sheet conveying apparatus disclosed in Japanese Patent Application Publication No. H02-18244, the conveying roller, which conveys the sheet material at the upstream of a resist roller, can be moved in a direction along the axis of rotation. The sheet conveying apparatus is adapted to correct the skew of a sheet material P by abutting the leading end of the sheet material P in a conveying direction T against a resist roller A. Further, a thrust force generated from the rotational force due to the warp of the sheet material P after the skew is corrected is removed by the movement of the conveying roller along the axis of rotation. This eliminates the aforesaid rotational force and solves the problem in that the sheet material P is skewed again after the correction of a skew.
Further, a plurality of conveying rollers that has moved by the thrust force is configured such that they can be reset to predetermined positions by being urged by coil springs from both sides in the axial direction (refer to Japanese Patent Application Publication No. H02-18244).
U.S. Pat. No. 7,533,878 discloses a solution to the warp of a sheet attributable to the correction of a skew. According to the solution, a feeding roller is supported by a movable arm member, and the nip (sandwiching) by the feeding roller and a pinch roller is cleared by moving the arm member.
In the sheet conveying apparatus disclosed in Japanese Patent Application Publication No. H02-18244 described above, a plurality of conveying rollers is always set such that they can be moved in the axial direction of a rotating shaft. This poses a problem in that the conveyance of a sheet material becomes unstable until the sheet material is abutted against a resist roller, which corrects the skew of the sheet material. Further, according to the sheet conveying apparatus disclosed in Japanese Patent Application Publication No. H02-18244, the conveying roller is integrally provided relative to the rotational direction of the rotating shaft. Hence, in the case where a relatively large correction amount of a skew of the sheet material is required, the warp of the sheet material may not be satisfactorily eliminated. For this reason, the sheet material P is required to be slipped relative to the conveying roller to some extent in order to fully eliminate the warp of the sheet material P that takes place between the conveying roller and the resist roller. On the other hand, however, it is necessary to minimize the slippage between the sheet material P and the conveying roller so as to obtain a sufficient conveying force for conveying the sheet material P. Satisfying these conflicting requirements is difficult.
As described above, according to the sheet conveying apparatus disclosed in Japanese Patent Application Publication No. H02-18244, it is difficult to achieve stable conveyance of a sheet material while restraining the sheet material from skewing at the same time. Thus, during the conveyance of the sheet material after the correction of the skew, differences are developed between both ends in the width direction of the sheet material in the conveying force and the conveying distance, leading to a skew of the sheet again. Applying the aforesaid sheet conveying apparatus to an image forming apparatus, especially a high image quality ink jet recording apparatus, would present a problem of uneven colors or uneven densities in an image recorded on a sheet material.
Further, the description of the construction disclosed in U.S. Pat. No. 7,533,878 provides no specific detail of the construction of a drive section for clearing the nip between the feeding roller and the pinch roller. There is a possible method in which, for example, the feeding roller is driven by an exclusive motor therefor, and the rotational direction of the motor is switch to control the movement of the arm member so as to clear the nip. This, however, would require the addition of a motor or a plurality of drive sections, resulting in a disadvantage of an increase in the manufacturing cost or an increased size of the apparatus.